Tensile behaviour of superelastic NiTi alloys charged with hydrogen under applied strain

Abstract

The tensile behaviour of NiTi alloys is investigated after hydrogen charging during the austenite, half-transformation and martensite phases. The specimens are charged with different current densities and charging durations. During the tensile tests, the strain of the plateau transformation decreases due to hydrogen-induced residual martensite variants. This decrease becomes important when the charging happens during the martensite phase. Accordingly, the hydrogen ensures the stability of the phase in which the charging process occurs. Moreover, a heightening of transformation stress is noticed during the plateau. The transformation stress increases when the current density grows and the charging duration rises. This occurrence is caused by the interaction between the hydrogen and NiTi structures, where hydrogen delays the NiTi martensite transformation. This paper is part of a thematic issue on Hydrogen in Metallic Alloys

Strain Rate Effect upon Mechanical Behaviour of Hydrogen-Charged Cycled NiTi Shape Memory Alloy

“…However, compared to the non-immersed alloy in the 0.9% NaCl solution, the NiTi archwire indicates arise in the martensite start stress of about 10 MPa. This phenomenon is attributed to the solid-solution hardening during hydrogen charging [ 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 ].…”

Finite element analysis of hydrogen effects on superelastic NiTi shape memory alloys: Orthodontic application

“…Table 2 presents all the values of material parameters needed to simulate the numerical response of the developed model. The parameters related to the hydrogen effects are calibrated from our previous experimental tests (Elkhal Letaief et al, 2016, 2017a, 2017b). The parameters f h and y h , which present the transformation delay and the dissipation induced by hydrogen, are obtained through one-cycle tensile tests after charging wire with hydrogen (Elkhal Letaief et al, 2016).…”

“…The parameters f h and y h , which present the transformation delay and the dissipation induced by hydrogen, are obtained through one-cycle tensile tests after charging wire with hydrogen (Elkhal Letaief et al, 2016). The chemical expansion coefficient β and the constants related to hydrogen-induced residual martensite ( C A , C M ) are obtained through in situ tests that consist in charging wires with hydrogen during applying tensile load (Elkhal Letaief et al, 2017a, 2017b).…”

“…Besides, it was demonstrated that hydrogen would induce an expansion of the NiTi structure and would provoke residual martensite variants. These results were obtained through in situ tests where the NiTi wires were charged with hydrogen during applying mechanical load (Elkhal Letaief et al, 2017a, 2017b; Yokoyama et al, 2017).…”

Hydrogen in metallic alloys

“…Two concerned hydrogen permeation in iron, a key aspect of the mechanism [5,6] albeit approaching the matter from the different directions of modelling and experiment, which should therefore provide a very useful perspective between them. Another two addressed both the mechanism and characterisation aspects [7,8], the former in martensitic steels, and the latter in superelastic NiTi alloys. The remaining papers addressed characterisation [3,9,10].…”